Because of its ready availability, low cost and potential for reducing particulate emissions, natural gas is gaining acceptance as a fuel for internal combustion engines. Natural gas is only one example of a gaseous fuel with such benefits that can replace diesel in internal combustion engines while achieving similar performance to diesel-fuelled engines with lower particulate matter and/or nitrogen oxide (NOx) emissions. While this disclosure relates to natural gas fuelled engines, other suitable gaseous fuels such as hydrogen, propane, ethane, butane, methane, and mixtures thereof can also be used as substitutes for diesel to achieve similar benefits.
Early approaches to fuelling internal combustion engines with natural gas mixed natural gas with the intake air prior to the introduction into the engine cylinder (a process known as “fumigation”). Because natural gas has a higher auto-ignition temperature than diesel, in these engines a pilot amount diesel fuel was injected directly into the combustion chamber near top dead center to trigger ignition of the natural gas. However, pre-mixing natural gas and air in this manner limits the amount of diesel that can be substituted with natural gas and/or the compression ratio that can be safely used, because fumigating too much natural gas can result in engine knock, which is premature detonation of the fuel that can damage the engine. Another disadvantage of fumigated engines is that under low load engine operating conditions, the pre-mixed homogeneous mixture of natural gas and air becomes too lean to burn. Consequently, engines that introduce natural gas by fumigation have not been able to match the power, performance, and efficiency of equivalently-sized diesel engines.
More recently an improved type of dual fuel internal combustion engine has been developed, herein referred to as a “high pressure direct injection” (HPDI) gaseous-fuelled engine. Herein “direct injection” is defined to mean injection of the fuel directly into the combustion chamber through nozzle orifices that communicate directly with the combustion chamber. That is, in an HPDI gaseous-fuelled engine, none of the fuel is injected into the intake manifold or intake ports. One approach to this type of engine uses a pilot amount of diesel fuel to trigger ignition of the gaseous fuel but the main difference is that the gaseous fuel is injected directly into the combustion chamber so that it burns in a combustion mode that is more similar to a conventional diesel engine, in which fuel is directly injected into the combustion chamber late in the compression stroke or early in the power stroke when the piston is near top dead center. Accordingly, with an HPDI gaseous-fuelled engine the amount of gaseous fuel that can be injected is not knock-limited because with the fuel being injected later in the engine cycle, there is no danger of premature detonation. Also, at low load operating conditions because the fuel is not pre-mixed, and instead burns in a diffusion combustion mode, the same challenges that fumigated engines have with these operating conditions are not encountered with HPDI gaseous-fuelled engines. Unlike fumigated engines, pilot-ignited HPDI gaseous-fuelled engines have been proven to achieve substantially the same combustion efficiency, power and torque output as state of the art diesel engines, while on average using natural gas for at least about 95% of the total fuel consumed on an energy basis. A challenge associated with the operational principle underlying pilot-ignited HPDI gaseous-fuelled engines is that means must be provided for injecting the gaseous fuel and the pilot fuel directly into the combustion chamber. Modern internal combustion engines can typically have two intake valves and two exhaust valves and these valves occupy a substantial amount of the space in the cylinder head above the combustion chamber. Consequently, it can be a problem finding enough space in the cylinder head to install a second fuel injector. A solution has been to design a dual fuel injector that provides two separate injectors in one body. Following this approach, the applicant has developed many different designs with concentric valves, wherein a smaller pilot fuel injection valve is positioned in the middle, with the body of the pilot fuel valve acting as the needle for a gaseous fuel injection valve, which is concentrically arranged in the annular space around the pilot fuel injection valve. Examples of these designs are disclosed in co-owned U.S. Pat. Nos. 6,073,862, 6,336,598, 6,761,325 and 7,124,959.
A problem observed with pilot-ignited HPDI engines employing concentric gaseous and liquid fuel injection valves is that there can be periodic variations in engine speed and NOx emissions. The amplitude of these variations can be particularly large when the load is small. These variable conditions can lead to irregular engine operation. Previously known concentric gaseous and liquid injection valves have had an inner needle valve that is free to rotate around its longitudinal axis. Advantages of this design include simplification which results in easier and lower cost manufacturing. Also, it was believed that it is best to allow the inner pilot fuel valve body to rotate freely to better adapt to the different characteristics of each cylinder in a multi-cylinder engine, and it was believed that a fixed interlace angle would be difficult to optimize for every cylinder. The experiments have shown that unstable engine speed can be attributed to variations in combustion quality that are believed to be caused by variations in the interlace angle especially when there is an equal number of gaseous and pilot fuel orifices. The interlace angle is defined as the angle between the axis of a gaseous fuel jet and that of the pilot fuel spray, neighboring the gaseous fuel jet, as viewed in the direction of the injection valve's longitudinal axis, which is a view that is typically depicted as a top or bottom view of the combustion chamber (that is, the view depicted in FIGS. 6 and 9, described later). Co-owned U.S. Pat. No. 6,439,192 teaches a solution to reduce such variations by using an injection valve that has an unequal number of pilot fuel and gaseous fuel orifices.
While the solution taught by the '192 patent might solve the problem for an engine with concentric gaseous and liquid fuel injection valves positioned in the center of the cylinder, it would not be a desirable solution for all engine configurations. For example, a different solution is needed for a concentric arrangement of gaseous and liquid fuel injection valves that is positioned off-center or in an inclined position. When an injector comprising a concentric arrangement of needle valves with a freely rotating inner needle valve, as described in the above patents, is placed in an off-center or in an inclined position with respect to the longitudinal axis of the combustion chamber, a freely rotating pilot fuel injection valve could introduce more variability in the engine performance caused by uneven distribution of fuel within the combustion chamber and the effect of interaction of the fuel jets with the combustion chamber walls, the piston or the cylinder head.
Because there is a desire to continually improve the performance of internal combustion engines, to increase efficiency, increase power, reduce emissions, and increase combustion stability and robustness, there is a need for new and different approaches to dual fuel injector design that would further reduce the variability of the combustion process taking place within the combustion chamber of a direct injection internal combustion engine.